Washington, October 2 : Using data from NASA's Spitzer Space Telescope, scientists have determined that hot spots near the shattered remains of an exploded star are echoing the blast's first moments.
According to Eli Dwek of NASA's Goddard Space Flight Center in Greenbelt, Maryland, and Richard Arendt of the University of Maryland, Baltimore County, these echoes are powered by radiation from the supernova shock wave that blew the star apart some 11,000 years ago.
"We're seeing the supernova's first flash," Dwek said.
Other Spitzer researchers discovered hot spots near the Cassiopeia A supernova remnant and recognized their importance as light echoes of the original blast.
Dwek and Arendt used Spitzer data to probe this hot dust and pin down the cause of the echoes more precisely.
Six knots of silicate dust near the remnant show temperatures between -280 degrees and -190 degrees Fahrenheit. Although this might seem frigid by Earthly standards, such temperatures are downright hot compared to typical interstellar dust.
Scientists have shown that the only event that could make the grains this hot is the powerful and short-lived pulse of ultraviolet radiation and X-rays that heralded the death of the star.
The flash was a hundred billion times brighter than the sun but lasted only a day or so.
"They've identified the precise event during the demolition of the star that produces the echo we see," said Michael Werner, the Project Scientist for Spitzer at NASA's Jet Propulsion Laboratory in Pasadena, California.
Light from the explosion reached Earth in the 17th century, but no one noticed. The Spitzer find gives astronomers a second chance to study the supernova as it unfolds.
Although the explosion originally escaped detection, its aftermath - a hot, expanding gas cloud known as Cassiopeia A (Cas A), is one of the best-studied supernova remnants.
The blast zone lies 11,000 light-years away in the constellation Cassiopeia.
Evidence for a flash associated with this "shock breakout" existed only in computer simulations until January 9, 2008.
That's when NASA's Swift satellite detected a 5-minute-long X-ray pulse from galaxy NGC 2770. A few days later, a new supernova - designated SN 2008D - appeared in the galaxy.
The infrared echoes from Cas A arise from dust clouds about 160 light-years farther away than the remnant.
The supernova's initial radiation pulse expands through space at the speed of light, then encounters the clouds and heats their dust grains. The dust, in turn, reradiates the energy at infrared wavelengths.
The breakout radiation took 160 years to reach the clouds and, once heated, the dust's infrared energy had to make up the same distance.
The researchers plan to use the echoes to paint an intimate portrait of the explosion, the star, and the immediate environment.